Recently, as demand of data communication and various communication services increase in a wireless communication system, various methods for increasing transmission capacity are suggested. One of those methods is working on heterogeneous network technique. The heterogeneous network indicates a communication system including base stations having diverse phases, cell coverage and characteristics.
FIG. 1 illustrates a heterogeneous network system.
As shown in FIG. 1, the heterogeneous network indicates a system which overlays and operates a macrocell 100 and one or more picocells 102, 104 and 106 of difference coverage sizes. While it is not depicted in the heterogeneous network of FIG. 1, the macrocell 100 may further include at least one femtocell.
In the heterogeneous network, the compact cells (the picocells 102, 104, and 106 or the femtocell (not shown)) in the macrocell 100 can increase the whole transmission capacity of the system by reusing transmission resources. For example, provided that the macrocell 100 can utilize the transmission band 10 MHz, when one compact cell having the transmission band of 10 MHz is installed, the transmission band of 20 MHz in total can be ensured ideally. That is, when N-ary compact cells are installed in one macrocell, the transmission band corresponding to N times of the existing transmission band can be ideally secured and thus the throughput can increase N times.
However, even when the N-ary compact cells are installed in one macrocell in the actual environment, the obtained transmission gain is smaller than the N-times throughput because of interference between the heterogeneous cells or irregular distribution of mobile stations. That is, in the actual heterogeneous network system, interference between the macrocell and the compact cell and interference between the compact cells can degrade the performance. As a plurality of mobile stations is irregularly distributed in each compact cell, some resources can be unused to thus degrade the performance.
FIG. 2 depicts the distribution of mobile stations in the heterogeneous network system.
As shown in FIG. 2, in the heterogeneous network including a plurality of picocells 211 through 213 in a cell 202 of a macro Base Station (BS) 200, a plurality of Mobile Stations (MSs) 220 through 224 each can select their serving cell by measuring a signal strength quality factor of the cells. Hence, the picocell A 210 and the picocell C 212 can allocate resources to the MSs 221 and 225 which select the picocells as the serving cell, whereas the picocell B 211 and the picocell D 213 are not selected as the serving cell by any MSs and thus cannot allocate the resource as shown in FIG. 2. As a result, while the transmittable resource amount increases thanks to the picocell B 211 and the picocell D 213, the actual throughput is not enhanced. As such, when there are many picocells not allocating the resource to the MS in the heterogeneous network, the whole transmission efficiency of the heterogeneous network is decreased.
In addition, although an MS accesses the picocell, the data amount to be transmitted by the MS is limited in the actual cellular environment. Thus, the unused transmission resource can generate in each picocell and the transmission efficient is deteriorated. In this regard, a cell selection method for increasing the transmission efficiency in the heterogeneous network is demanded.